1. Field of the Invention
This invention generally relates to a color printer for printing color images by using the electrophotographic process, and more particularly to a color printer in which toners of different colors are used for printing.
2. Description of the Related Art
In a typical color printer, for example, first to fourth printing units are arranged on the paper feeding path to print the same image in different colors such as yellow, cyan, magenta and black. In each of the printing units, an image carrier is uniformly charged and the image carrier surface is scanned by an exposure beam so as to form an electrostatic latent image on the image carrier and then toner of a corresponding color is supplied to the image carrier to develop the electrostatic latent image as a toner image. The paper is fed along the paper feeding path to be sequentially supplied to the first to fourth printing units. Each of the printing units transfers the toner image on the same area of the paper supplied thereto. Thus, a color image will be formed by combining the toner images of yellow, cyan, magenta and black by the time the paper passes all of the first to fourth printing units and is discharged.
The first to fourth printing units are generally formed with the same structure so as not to cause a variation in the printing quality and perform the printing operation on the same process condition. For this reason, for example, the charging voltages are set to the same values in the respective printing units. Further, as the toners of different colors, materials capable of permitting a sharpest color image to be produced on the above process condition are selected.
In a case where a large number of color images are to be printed but only a small amount of toner remains, it is sometimes required to suppress the consumption of toner without seriously thinking that the printing concentration becomes low. If the amount of toner becomes insufficient during the printing operation, it becomes necessary to interrupt the printing operation and quickly replace the toner packs. In this case, the above replacing operation can be effected only when a spare toner pack is prepared at hand. In addition, in some cases, it is required to positively reduce the printing concentration of each color image because of the service object of the printing matter.
As a method of suppressing the consumption of toner, it is considered to reduce the powers of exposure beams used in the above printing units by a constant amount, for example. However, in this case, the color balance of the color image cannot be maintained and the contrast is liable to be lowered. Particularly, a yellow portion tends to be blurred and the image of this portion becomes unclear. That is, the image cannot be lightly printed without degrading the printing quality.